Rabbits are helping scientists understand the mysterious female orgasm

As ingenious as humans are, there’s always evolution to put us in our place. Just as we descended from a bag with teeth, the peak of sexual pleasure, the orgasm, has equally visceral evolutionary routes.

On Monday, scientists added yet another chapter to the storied evolutionary history of the orgasm, thanks to a rabbit called “Frank” and his female companions.

A paper in Proceedings of the National Academy of Sciences suggests that the human orgasm has evolutionary roots deep in the brain that stretch back thousands of years. Back in the day, they suggest that the female orgasm was actually a crucial tool to help species reproduce, a function that the current female orgasm no longer serves.

Study co-author Günter P. Wagner, Ph.D., a professor of evolutionary biology at Yale tells Inverse that this finding totally changes the way that scientists think about female orgasms. If it’s no longer one way to help species reproduce why does it exist? Pleasure, alone it seems, isn’t a sufficient answer. Wagner suggests it’s a more complicated story:

“Thus the question shifts from ‘how did female orgasms evolve’ to ‘why is [the] female orgasm maintained, given that it lost its function in reproduction [ovulation]?’” he tells Inverse. “It also shows, with the work of other researchers, that there is no correlation between female orgasms and reproductive success; that the female orgasm has to have some other, non-reproductive role.”

A Short Evolutionary History of the Female Orgasm

From an evolutionary standpoint, scientists have become frustrated with the female orgasm. To have persisted over the course of evolution it had to have had some benefit, but scientists couldn’t figure out what that advantage was, at least at first.

One early idea posited by Wager and his co-author Mihaela Pavlicev, Ph.D., (the lead author on this paper) pointed towards a theory called the Ovulation Homolog Theory. That theory links orgasms and ovulation, which is the release of an egg from the ovaries.

In monkeys, rats, mice, sheep and humans, this happens each month, which is sometimes called spontaneous ovulation. In other animals, like rabbits or camels, sex literally triggers ovulation, and the orgasm may be what opens the ovulation floodgates, as Wagner suggested in his 2016 paper. Wagner explains that they’re not sure exactly why humans developed their workaround, but that it might have to do with the way that different animals live:

“There is no firm answer,” says Wagner. “So my guess is that in social animals, where copulation partners are around all the time, natural selection reduced female fertility to avoid overpopulation and thus degradation of the local environment.”

Back when they first defined that theory in 2016, that suggested that the current female orgasm traces goes back to when rabbits and humans shared a common ancestor all those years ago. At that time, its purpose would have been clear: it helped those creatures reproduce.

In this experiment, Wager and Pavlicev tested that theory and argue that they found proof of that tight evolutionary history between the female orgasm and ovulation.

How Copulating Rabbits Uncovered The Orgasm’s Deep Roots

In a series of experiments, Wagner and the team injected female rabbits with a serotonin reuptake inhibitors (SSRI), called fluoxetine. These drugs are classic antidepressants but in humans, and animals, can also prevent orgasms from occurring. Not that, from this paper, it is unclear if rabbits orgasm in the first place.

The idea was that since rabbits ovulate once they have sex — each one with the same male rabbit “Frank” — this would be a good way to test whether the orgasm itself was actually related to ovulation. In other words, if same thing that stops humans from orgasming, also decreases rabbit ovulation, then perhaps there’s some deep connection between the two that was once a part of a shared evolutionary history.

When the female rabbits were given SSRIs they reduced their ovulation post-copulation by 30 percent. That suggested that there was an underlying architecture between rabbit ovulation and human orgasm — particularly in the brain.

Firstly, says, Wagner it showed that there is a connection between ovulation and orgasms. But at least in humans this connection has been lost over the years when we developed spontaneous ovulation, which is basically, a workaround. But Wagner and Pavlicev argued that freed up the female orgasm to perform another function.

That finding, he adds, implies that there has to be something else about the female orgasm in humans; another non-reproductive advantage that has developed since we split off from other mammals and has allowed it to survive.

“In particular the implication is that female orgasm has likely no function in reproduction, but probably still exists because of the benefit of the women herself, and other primate females, that have a similar biology,” he says.

“The question now is: “what is that non-reproductive functional role of female orgasm?” We do not have answers for that yet,” he continues.

That new chapter of the female orgasm has yet to be uncovered.

Abstract: The ovulatory homolog model of female orgasm posits that the neuro-endocrine mechanisms underlying female orgasm evolved from and are homologous to the mechanisms mediating copulation-induced ovulation in some mammals. This model predicts that pharmacological agents that affect human orgasm, such as fluoxetine, should also affect ovulation in animals with copulation-induced ovulation, such as rabbits. We tested this prediction by treating rabbits with daily doses of fluoxetine for 2 wk and found that fluoxetine treatment reduces the number of ovulations postcopulation by 30%. In a second experiment we tested whether this result was mediated by an effect on the brain or via peripheral serotonin functions. We treated animals with fluoxetine and induced ovulation with a single injection of human chorionic gonadotropin. In this experiment ovulation rate was nominally reduced by only 8%, which is statistically not significant. We conclude that the effect of fluoxetine on copulation-induced ovulation rate supports the ovulatory homolog model of female orgasm, suggesting that female orgasm has very deep evolutionary roots among the early eutherian mammals.